The present invention is related to a method for ultrafast magnetization reversal with small applied magnetic fields. More particularly the invention relates to magnetic recording.
Magnetization reversal is an elementary process underlying key technologies of our civilization such as electric transformation or magnetic recording. In conventional magnetization reversal as practiced today the reversing magnetic field is applied antiparallel to the magnetization direction. Therefore, the reversal speed is limited to a time scale which is at the nanosecond level.
Magnetic recording is an interdisciplinary field involving physics, material science, communications, and mechanical engineering. The physics of magnetic recording involves studying magnetic heads, recording media, and the process of transferring information between the heads and the medium.
Many magnetic recording systems, which are adaptable for recording and storing data, are known. Conventional systems employ a magnetizing pattern on the surface of a magnetic recording medium The magnetic medium has a magnetizing direction or a premagnetization whereby the pattern of magnetization is formed along the length of a single track, or a number of parallel tracks. The medium is in the form of a magnetic layer supported on a nonmagnetic substrate. Recording or writing takes place by causing relative motion between the medium and a recording transducer, also referred to as recording head. In general, the recording head is a ring-shaped electromagnet with a gap at the surface facing the medium. When the head is fed with a writing current representing the signal to be recorded, the fringing field from the gap magnetizes the medium, respectively. The recorded magnetization creates the above-mentioned pattern, that is in the simplest case a series of contiguous bar magnets. A xe2x80x9cone bitxe2x80x9d corresponds to a change in current polarity, while a xe2x80x9czero bitxe2x80x9d corresponds to no change in polarity of the writing current. A moving disk is thus magnetized in the xe2x80x9c+xe2x80x9d direction for positive current and is magnetized in the xe2x80x9cxe2x88x92xe2x80x9d direction for negative current flow. In other words, the stored xe2x80x9conesxe2x80x9d show up where reversals in magnetic direction occur on a disk and the xe2x80x9czeroesxe2x80x9d reside between the xe2x80x9cones.xe2x80x9d
A variety of magnetic media have been used for magnetic recording over the years. However, most modern magnetic media use a thin layer of ferromagnetic material supported by a non-magnetic substrate. The magnetic layer can be formed of magnetic particles in a polymer matrix. Alternatively, the layer can be a vacuum deposited metal or oxide film The use of a thin magnetic layer permits many possible configurations for the substrate. Magnetic media are differentiated into xe2x80x9chardxe2x80x9d and xe2x80x9csoftxe2x80x9d media. Hard media require large applied fields to become permanently magnetized. Once magnetized, large fields are required to reverse the magnetization and erase the material. Such media, with large saturation and high coercivity are appropriate for such applications as computer data storage. Soft media, on the other hand, require relatively low fields to become magnetized. These materials are more appropriate for applications such as audio recording. The choice of the media influences the way the magnetization is recorded on the medium. This is because the direction of the recorded magnetization is strongly influenced by the magnetic anisotropy of the used medium. Thus, different techniques in recording exist, for example, longitudinal recording in which the magnetization direction is directed along the length of the track or perpendicular recording whereby the medium shows perpendicular anisotropy. Media with needle shaped particles oriented longitudinally tend to have a higher remanent magnetization in the longitudinal direction, and favor therefore longitudinal recording. This longitudinal orientation can then be supported by an appropriate head design, e.g. a ring head, which promotes longitudinal fields. Longitudinal recording is today""s most applied and used technique. Nevertheless, a medium can also be constructed perpendicularly to the plane of a film Such media have a higher remanent magnetization in the perpendicular direction, and favor perpendicular recording. This perpendicular orientation can be supported by a head design, e.g. a single-pole head, which promotes perpendicular fields. Perpendicular recording media are generally recognized as supporting more stable high-density recording pattern than longitudinal media.
U.S. Pat. No. 5,268,799 is related to a magnetic recording and reproducing head that records a signal into and reproduces a signal from a magnetic recording medium having a perpendicularly magnetizable Mm The magnetic recording and reproducing head includes a magnetic sensing section comprising a slender needle of a soft magnetic material, and an exciting coil wound around the slender needle for magnetizing the slender needle to record a signal on the magnetic recording medium To reproduce the recorded signal high-frequency electric energy is applied to the magnetic sensing section to produce a reflected wave, and a change in the reflected wave caused by a leakage magnetic field produced by a signal recorded on the magnetic recording medium is detected as representing the recorded signal.
C. H. Back et al. describe in their article xe2x80x9cMagnetization Reversal in Ultrashort Magnetic Field Pulsesxe2x80x9d, Physical Review Letters, Vol. 81, 3251 (1998), an experiment for studying magnetization reversal in perpendicularly magnetized Co/Pt films, whereby a short but strong magnetic field pulse is used. The applied magnetic field pulse is very strong and therefore not suitable for magnetic recording. Furthermore, a magnetic recording head is not able to generate such a strong, high energetic pulse.
Today""s computers store data on magnetic disks in the form of binary digits or bits. Such a disk is rotating when the data are transmitted to the disk drive and processed in a corresponding time sequence of binary xe2x80x9conexe2x80x9d and xe2x80x9czeroxe2x80x9d digits, or bits. Typical data rates today are about 30 MB/sec. This corresponds to magnetic-field pulses of 4 ns duration for recording. The current technologies apply antiparallel magnetic fields or magnetic-field pulses in order to reverse the magnetization direction.
Since the load of data which has to be stored increases dramatically, there is a need for faster operation in recording processes. Thus, the operating speed of the data storage systems is increasing. Today""s systems show some drawbacks, e.g. the speed is physically limited, and are hence not suitable for new generations. With the conventional technology the reversal speed is in the nanosecond time scale. Therefore a much faster technology is required.
It is an object of the present invention to overcome the disadvantages of the prior art.
It is another object of the present invention to provide a concept for high data rate recording.
It is still another object of the present invention to provide a method of performing ultrafast magnetization reversal.
It is a further object of the present invention to provide a method for ultrafast magnetic recording.
It is still a further object of the present invention to provide a device, a medium, and a system for ultrafast magnetic recording.
The objects of the invention are achieved by the features of the enclosed claims. Various modifications and improvements are contained in the dependent claims.
The underlying concept of the present invention concerns ultrafast magnetization reversal in an in-plane magnetized layer having a magnetization For achieving ultrafast magnetization reversal a small and short external magnetic field or field pulse is applied approximately perpendicular to the magnetization of the layer such that the magnetization precesses around the external magnetic field. The external magnetic field is only maintained until the precession suffices to effect the magnetization reversal, that means in the simplest case until the magnetization turns out of plane to about 20xc2x0. Then, the combination of the layer""s demagnetization field and anisotropy field completes the reversal process, and turns the magnetization in the opposite direction. The magnetization turns into the opposite direction without the external magnetic field. Furthermore, the external magnetic field can be maintained, whereby the magnetization rotates around the plane for a while, and switched off such that the turn of the magnetization stops at a multiple of xcfx80, preferably in the antiparallel or opposite direction which means an odd multiple of xcfx80. The external magnetic field, on the other hand, should be short enough to avoid relaxation of the magnetization into its direction. In a uniaxial in-plane magnetized layer the magnetization shows two stable states in the plane, that is either parallel or antiparallel.
The external magnetic field is comparable to an in-plane anisotropy field of the layer and sufficient to reverse the magnetization, provided the field is applied at about a right angle to the magnetization in order to exert maximum torque on the magnetization or spins. This fact shows the advantage that a small magnetic field is enough to induce magnetization reversal whereby less energy for the creation of said magnetic field is sufficient.
It is an advantage of the present invention that a much faster magnetization reversal can be achieved since it seems that no fundamental limit exists for the time of reversal. This ultrafast magnetization reversal, for instance, can be utilized for magnetic recording. High data rate recording, much faster than 30 MB/sec, becomes feasible and allows to store an increasing load of data The present invention improves conclusively the technology of data storage and can be utilized for longitudinal recording or perpendicular recording.
When the external magnetic field is applied at an angle so that a maximum torque is exerted on the magnetization, then the advantage occurs that the physical effect of the ultrafast magnetic reversal can be exploited at best. This can be achieved if the external magnetic field is applied essentially perpendicular to the magnetization.
It is also advantageous if the external magnetic field can be applied at an angle between 45xc2x0 and 135xc2x0 to the magnetization, because then the external magnetic field does not need to be aligned exactly.
If the applied external magnetic field is stronger than a magnetic anisotropy field of the in-plane magnetized layer, and this means in particular that the external magnetic field needs only to be slightly stronger than the magnetic anisotropy field in order to induce an ultrafast magnetic reversal process, then the advantage occurs that the ultrafast magnetic reversal process can be initiated by a relative weak external magnetic field. This field is creatable by a device or a recording head that requires not much power, i.e. the power consumption of a storage system can be held at low level. This is especially advantageous for portable computers which use a battery.
When the applied external magnetic field is ceased before the magnetization aligns in the direction of the external magnetic field, then the advantage occurs that the magnetization turns in the opposite direction according to the present invention and not in the direction of the applied external magnetic field.
It is advantageous if the applied external magnetic field has a small field amplitude. Since the external magnetic field is only used to lift the magnetization out of the plane and the perpendicular component of the magnetization gives rise to a demagnetization field, a subsequent damped precession around the layer""s demagnetization field and anisotropy field completes then the reversal process. In fact it is the demagnetization field which makes that the external magnetic field necessary for magnetization reversal is so small. Thus, already less power is sufficient to create such an external magnetic field and further the adjacent ranges on a storage medium are not disrupted or influenced by the external magnetic field. For a Co film, as shown in experiments and described below, a field amplitude of  less than 185 kA/m at a pulse length of 2 ps (half width at half amplitude) is sufficient.
When the external magnetic field is applied at the picosecond time scale, e.g. between 1 ps and 1000 ps, then the advantage occurs that the recording process can be ultrafast whereby best magnetization-reversal results can be achieved. Therefore, much more data or a much larger load of data can be recorded and stored. An appropriately designed magnetic-field generator, e.g. as part of a recording head, that is able to generate such short magnetic fields or pulses should be used.
The magnetization of a uniaxial in-plane magnetized layer has two stable states; either the magnetization is directed in one direction, i.e. parallel, or in the opposite direction, i.e. antiparallel, in the plane. Therefore, the rotation of the magnetization stops at a multiple of xcfx80 or in one of the two directions of the plane after the external magnetic field is ceased.
When the precession of the magnetization around the layer""s demagnetization field and anisotropy field completes the magnetization reversal, then the advantage occurs that the external magnetic field can be ceased before the entire magnetization reversal process is finished. This helps to save energy and increases the recording speed because the recording head can be moved already to its next position after a magnetization reversal is induced.
The external magnetic field can be applied in the plane of the layer or perpendicular to the plane. This brings the advantage that the external magnetic field is useable from various directions. Another advantage is that conventional recording heads could be used at an angle of about 90xc2x0, provided that the recording heads are designed such that these are able to create a short magnetic field or field pulse for inducing ultrafast magnetization reversal.
It is advantageous if the in-plane magnetized layer comprises nanoparticles, e.g. crystal grains, preferably identical grains, or single-domain particles, and has a demagnetization factor close to 1, because then the magnetic field required to reverse the magnetization direction can be decreased. Since every magnetic material shows a damping constant xcex1, the material for the layer should be selected such that it shows a low damping constant xcex1. Furthermore, the damping constant a and therefore the magnetic material for the layer can be adapted accordingly to achieve ultrafast magnetization reversal with best results. In general, it is the combination of material parameters, depending on the magnetization, the magnetic anisotropy field, the damping constant, and an external-field value at a given pulse duration which determine the efficient functioning of the reversal process.
In principle, today""s storage disk materials are suited for ultrafast magnetization reversal, but improvements in material science will provide more suited materials for ultrafast magnetization reversal.
An in-plane magnetized medium or layer can be part of a flexible disk, a hard disk, a tape, or any other device capable to reverse its magnetization for recording and storing data.
The following are informal definitions to aid in the understanding of the description.
{right arrow over (M)}xe2x80x94magnetization, which indicates the alignment of spins
Msxe2x80x94saturation magnetization; e.g. for Co at room temperature MS=1.7 T
{right arrow over (H)}ex xe2x80x94external magnetic field
{right arrow over (H)}A xe2x80x94magnetic anisotropy field
{right arrow over (H)}Dxe2x80x94demagnetizing field
xcex1xe2x80x94damping constant
∂xe2x80x94angle between {right arrow over (M)} and {right arrow over (H)}
"THgr"xe2x80x94angle between {right arrow over (M)} and the plane of a layer
xcexc0xe2x80x94permeability of vacuum